One technique for cutting workpieces such as metal panels and high performance composite panels used as air frame components involves the use of fluid abrasive cutting systems. These systems employ an injector nozzle which dispenses a liquid, such as water, entraining an abrasive material at extremely high pressures. The operating pressures of such systems normally range from 30,000 to 60,000 psi or higher. In a typical application, the high pressure liquid flowing through the nozzle induces a vacuum in a supply line leading to a source of an abrasive grit such as garnet, silica, alumina or the like. Typical abrasive constituents include 100 mesh abrasive particles for cutting composite materials such as composite laminates of graphite-epoxy or Kevlar Fiber reinforced resins and 60-80 mesh abrasive particles for cutting metals such as titanium and aluminum.
The high pressure jet stream passing through the cut or "kerf" formed in the workpiece normally retains a substantial kinetic energy necessitating that the stream be caught in a catcher vessel in a manner to dissipate the remaining energy. Various systems have been proposed for the control of the high pressure nozzle and an associated catcher vessel as the cutting head moves relative to the workpiece being cut. One relatively simple arrangement employed an articulated beam and swivel arrangement for movement of the cutting head through x, y and z axes is disclosed in U.S. Pat. No. 4,435,902 to Mercer, et al. In Mercer, a high pressure jet nozzle and a catcher vessel are interconnected by a U-shaped tube which extends around the side of the workpiece being cut. The catcher is equipped with an impingement disk to absorb the kinetic energy of the jet stream passing through the workpiece and into a catcher tube in the catcher body. The catcher tube can be moved up and down relative to the catcher body to provide a desired distance between the cutting nozzle and the catcher orifice to accommodate workpieces of different thicknesses. The catcher or the nozzle can optionally be equipped with pins to follow a track or guide which may be attached to the workpiece or work surface.
An alternative arrangement for absorbing the kinetic energy from a high pressure jet cutting stream is disclosed in U.S. Pat. No. 4,532,949 to Frank. In the Frank device, the energy absorber is in the form of a fluid flow conduit mounted in an adjustable support assembly which in turn is secured to the cutting head assembly in which the nozzle is mounted. In the Frank system, a carbide ball is mounted in a ball seat below the catcher orifice so that it is impinged by the jet stream entering the catcher chamber.
A substantially more sophisticated system for robotically controlled abrasive jet cutting is disclosed in Earle, III, George A., "Automatic Trimming of Composite Panels", SAE Paper No. 861,675, October 1986, Society of Automotive Engineers. As disclosed there, the robotic cutting system involves a cutting head which is moved relative to a workpiece by operation of a sixaxes gantry robot system. The workpiece to be cut is placed on a suitable support surface and the location of the workpiece relative to the gantry system is accurately determined by a visual control system which senses targets in the workpiece and makes appropriate changes in the program matrix to accommodate the actual position of the workpiece. The cutting head can be moved under the control of a central controller through a three axes cartesian coordinate system to arrive at the desired location. Movement through pitch and yaw axes can then be employed to arrive at the desired orientation of the cutting nozzle relative to the workpiece surface to be cut. A catcher vessel is supported from the cutting head a fixed distance from the nozzle by means of a bracket assembly which extends around the edge of the workpiece being cut. The catcher vessel is filled with stainless steel balls which act through erosion and mobilization of the balls to dissipate the kinetic energy of the spent cutting stream. The cutting head can be moved around a sixth axis coincident with the cutting axis of the nozzle in order to avoid impacting the bracket assembly upon the workpiece.